In the scheme of emission control for diesel engines, reduction of PM is as important as reduction of NOx. DPFs are known as effective means to reduce PM.
A DPF is a PM collecting device using a filter. When an engine is operating in a state where exhaust gas temperature is low, PM continues to accumulate on the DPF. Therefore, forced regeneration is performed to burn the PM (or soot in the PM) by forcibly raising the temperature.
In the DPF forced regeneration, late post-injection in which fuel is injected into cylinders (injection timing is so late that the fuel is not burned in the cylinders) is performed, and oxidation reaction is caused by an oxidation catalyst (hereafter, abbreviated as DOC) arranged upstream of the DPF, so that heat obtained by this reaction is utilized to keep the exhaust gas temperature in the DPF high enough (600 to 650° C.) to burn the soot deposited on the DPF.
In general, a late post-injection amount is subjected to feedback control such as PID control so that the temperature in the DPF is controlled at a target temperature. The target temperature is determined based on a DPF inlet gas temperature, a DPF outlet gas temperature, or a DPF internal temperature (these temperatures are referred to as the DPF temperature).
However, when the DPF temperature is controlled at a fixed value of the target temperature, problems as follows may be induced.
When the target temperature is set high, the temperature may possibly rise excessively when the soot deposited on the DPF is burned. For example, when the engine is idling, a state called “drop to idle” is produced, in which temperature tends to rise excessively. If a large amount of soot is deposited in this “drop to idle” state, the DPF internal temperature tends to rise rapidly and excessively.
When a critical soot deposition amount is defined as a soot deposition amount at which a DPF catalyst reaches a temperature (about 800 to 900° C.) at which it degrades in the “drop to idle” state, the DPF inlet temperature and the critical soot deposition amount assume a relationship as shown in FIG. 14. It can be seen, from this relationship, that as the soot deposition amount becomes greater, the DPF target temperature must be set to a lower value.
Particularly, in an initial stage of DPF regeneration in which the soot deposition amount is large, the risk of excessive temperature rise of the DPF is increased if the DPF temperature is high.
In contrast, if the target temperature is set low, the time required for regeneration is increased, and hence the risk is increased that late post-injected fuel falls from cylinder inner walls into an oil pan, causing oil dilution. FIG. 15 shows time variation of soot deposition amount during a regeneration process. It can be seen that the regeneration time becomes longer as the DPF regeneration temperature becomes lower.
It is known that the DPF target temperature can be controlled by varying the same based on some parameter. For example, Japanese Patent Application Publication No. 2007-239470 (Patent Document 1) describes that a target value of DPF inlet temperature is determined based on any one of soot deposition amount, change rate of soot deposition amount, DPF temperature, change rate of DPF temperature and the like.
Japanese Patent Application Publication No. 2009-138702 (Patent Document 2) describes that a target value of DPF inlet temperature is set according to the time elapsed from the start of forced regeneration of the DPF, and the target value is set lower as the time elapsed from the start of forced regeneration of the DPF becomes shorter. Furthermore, Japanese Patent Application Publication No. 2010-071203 (Patent Document 3) is also known as a document disclosing a technique to set the DPF inlet temperature target value.    Patent Document 1: Japanese Patent Application Publication No. 2007-239470    Patent Document 2: Japanese Patent Application Publication No. 2009-138702    Patent Document 3: Japanese Patent Application Publication No. 2010-071203
According to the technologies described in Patent Documents 1 to 3 in which regeneration conditions such as late post-injection amount are controlled so that the DPF temperature becomes a target DPF temperature, excessive rise of temperature is prevented by setting the target DPF temperature lower as the time elapsed from the start of forced regeneration becomes shorter.
However, when the control is performed based on a DPF temperature, the control cannot cope with significant change in temperature characteristics of the soot regeneration amount per unit time. This means that, the DPF internal temperature is raised by burning soot (by regeneration with O2).
Therefore, a relationship between soot regeneration amount per unit time and DPF temperature when the soot deposition amount is fixed is represented by a trend as shown in FIG. 16. As shown in FIG. 16, the soot regeneration amount per unit time changes with respect to the DPF temperature not linearly but exponentially. Therefore, the soot regeneration amount per unit time is increased significantly as the DPF temperature rises, whereby the risk that rapid and excessive rise of temperature occurs is increased. For example, the soot regeneration rate when the DPF temperature is 630° C. is about twice as high as when it is 600° C.
Thus, when the control is performed based on DPF temperature, the soot regeneration amount per unit time cannot be detected correctly, which has a risk of excessive rise of temperature.
The soot regeneration amount per unit time and the soot regeneration rate can be calculated by equations (1) and (2) below. The relationship between soot regeneration amount per unit time and DPF temperature as shown in FIG. 16 can be obtained from these equations (1) and (2).Soot regeneration amount per unit time [g/s]=soot regeneration rate [1/s]×soot deposition amount [g]  (1)Soot regeneration rate [1/s]=A×exp(−B/RT)×QO2γ  (2)where A, B, and γ each denote a constant, R denotes a gas constant, T denotes a DPF temperature [K], and QO2 denotes an O2 flow rate [g/s].